This invention relates to the method of removal of light impurities from caprolactam by distillation with water. The crude caprolactam (epsilon-caprolactam) is obtained from the Beckmann rearrangement of cyclohexanone oxime and must be purified for suitable use as a monomer to prepare polycaprolactam (nylon 6). See pages 425-436, Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 18, 3d Ed. 1982 (John Wiley) hereby incorporated by reference.
The following are definitions of the terms used in this patent application. By portion is meant 2-98%, preferably 5-90% of a process stream. By low boiling is meant those impurities boiling between the atmospheric boiling point of caprolactam and water. By low water content is meant a content below 10% by weight of water. By crude caprolactam is meant caprolactam with impurities from the process and includes mother liquor from the crystallizer of the caprolactam process. The process used for the invention may be batch or continuous. The crystallization used in the process herein can be single or multistage. Also the distillation used in the process herein can be single or multistage.
Many methods to purify crude caprolactam and other lactams are known. See column 1 of U.S. Pat. No. 3,347,852 hereby incorporated by reference. For a water crystallization process, fractional crystallization or solvent extraction may be used in the crude caprolactam or mother liquor as in U.S. Pat. No. 2,817,661 or U.S. Pat. No. 3,761,467, both hereby incorporated by reference. Multistage centrifuges and freezer crystallizers may be used as in U.S. Pat. No. 2,813,858, hereby incorporated by reference. Crystallization may be used from special solvents as in U.S. Pat. No. 3,966,712, hereby incorporated by reference. Also, solvent may be removed from crystallized lactam by distilling in the presence of water as in U.S. Pat. No. 4,148,793, hereby incorporated by reference.